1. Field of the Invention
Treating wood pulp with oxygen.
2. Review of the Prior Art
The following definitions will be used in this application.
Pulping is a changing of wood chips or other wood particulate matter to fibrous form. Chemical pulping requires cooking of the chips in solution with a chemical and includes partial removal of the coloring matter such as lignin associated with the wood.
Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.
Pulp quantity is expressed in several ways.
Oven dry pulp is considered to be moisture free or done dry. Its value is determined by drying the pulp in an oven at a temperature of 100.degree. to 105.degree. C. until it reaches constant weight. It usually is considered to have reached constant weight after 24 hours in the oven.
Air-dry pulp is assumed to have a ten percent moisture content. One air-dry ton of pulp is equal to 0.9 oven-dry tons of pulp.
There are two principal types of measurements to determine the completeness of the pulping or bleaching process, the degree of delignification and the brightness of the pulp. There appears to be no correlation between the two because the delignification factor is a measure of residual lignin within the pulp and the brightness is a measure of reflectivity of the pulp sheet.
The degree of delignification is normally used in connection with the pulping process and the early bleaching stages. It tends to be less precise when only small amounts of lignin are present in the pulp.
The brightness factor is normally used in connection with the bleaching process because it tends to be less precise when the pulp is dark and its reflectivity is low.
There are many methods of measuring the degree of delignification but most are variations of the permanganate test.
The normal permanganate test provides a permanganate or K number which is the number of cubic centimeters of tenth normal potassium permanganate solution consumed by one gram of oven dried pulp under specified conditions. It is determined by TAPPI Standard Test T 214.
The Kappa number is similar to the permanganate number but is measured under carefully controlled conditions and corrected to be the equivalent of 50 percent consumption of the permanaganate solution in contact with the specimen. It is able to give a degree of delignification of pulps through a wider range of yields than does the permanganate number. It is determined by TAPPI Standard Test T-236.
There are also a number of methods of measuring pulp brightness. It usually is a measure of reflectivity and its value is expressed as a percent of some scale. A standard method is GE brightness which is expressed as a percentage of a maximum GE brightness as determined by TAPPI Standard Method TPD-103.
Pulp yield may be measured in two ways. The first is the amount, by weight, of carbohydrates and lignin returned per unit of wood. Screened yield is closely related and proportional to this chemical return. A high screened yield means the chemical return is high and a low screened yield means the chemical return is low. The second measurement of yield is fiber yield, by weight, per unit of wood. Rejects or screenings are related to and inversely proportional to the fiber yield. A high reject level means there is a low fiber return and a low reject level means there is a high fiber return. The total yield is the sum of these two yields. The ideal situation would be one in which there is a high chemical return and a high fiber return indicated by a high screened yield and low screenings.
There is a great deal of art which describes both oxygen bleaching of pulp or refining of pulp. Consequently, the present discussion of the prior art will not be exhaustive but will consider only those patents and articles which appear to be quite pertinent to the present application.
The first of these is Laakso U.S. Pat. No. 4,002,528, which issued Jan. 11, 1977. This patent describes one environment for the present invention--two refiners 34 and 35 within the blow line 32 between digester 24 and a storage tank 38. This environment is usually encountered in a linerboard mill. The digester in a bleached pulp mill normally would not have refiners in the blow line.
The following patents are exemplary of those describing various oxygen treatment systems.
Grangaard, et al. U.S. Pat. No. 3,024,158, which issued Mar. 6, 1962 discloses the oxygen treatment of pulp to minimize brightness reversion. FIGS. 1 and 2 disclose a two-vessel system in which oxygen is added to the liquor in one vessel and the pulp treated with the oxygenated liquor in a second vessel.
Grangaard, et al. state that time of the reaction depends upon the temperature of the reaction. The time may be varied between 5 minutes to 3 hours at the reaction temperatures of between 100.degree. and 160.degree. C. Although the process may be used on unbleached kraft and other pulps of low brightness, it is preferable to use it with bleached pulp. Examples 1-8 of this patent describe treatment of unbleached kraft pulps. The time of treatment was 60, 120, and 180 minutes.
The patent also describes two ratios that are important when using oxygen and the parameters of these ratios. The first is the ratio of the oxygen pressure in the atmosphere in contact with the solution to the vapor pressure of the solution at the reaction temperature. It should be at least 0.35 and preferably 0.5 or more. The second is the ratio of the surface area of the solution in contact with the oxygen-containing atmosphere in square feet to the volume of the solution in cubic feet. It must be greater than 4.
FIG. 3 of the patent discloses an in-line system in which pulp is passed through a heat exchanger 23 and then continuously fed through a turbo mixer 24 while oxygen under pressure of at least 40 pounds per square inch is introduced. The treated pulp is discharged into a stock chest 22.
The Kamyr blow line oxygen system is described in Richter, U.S. Pat. No. 3,963,561 which issued June 15, 1976, and in Kleppe et al "Oxygen Alkali Delignification at Kamyr Digester Blow Line Consistency--a status report," 1976 International Pulp Bleaching Conference, May 2-6, 1976, TAPPI November 1976, Vol. 59, No. 11, pps. 77-80. In this system, oxygen is added to the pulp in the blow line between the digester and the oxygen reactor. The oxygen is added just prior to the refiner at the bottom of the reactor. The pulp is at a consistency of 5-20 percent and preferably 8-12 percent. The reactor is of an upflow-downflow type in which the pulp and oxygen are carried upward in a conical inner section of the reactor and flow downward in the outer section of the reactor. The oxygen reacts with the pulp in the upflow section of the reactor where the pulp must remain for from 20-30 minutes. During this portion of the cycle, the pulp is 90 percent oxidized. In the pilot plant the retention time in the inner section of the reactor was 40 minutes.
There are a number of mechanical features within the reactor to keep the pulp from floating to the top and to ensure that the pulp remains within the conical inner section of the reactor the appropriate length of time. The patent is directed to the reuse of the excess oxygen within the system.
The article describes the system in use at the Moss Norway plant.
Another approach is suggested in the International Paper patents to Roymoulik et al, U.S. Pat. No. 3,832,276, which issued Aug. 27, 1974, and to Phillips, U.S. Pat. No. 3,951,733, which issued Apr. 20, 1976.
The process requires a pulp at a consistency of less than 10 percent, preferably about 2 to 6 percent and most desirably between 3 and 4 percent. The pulp is mixed with oxygen is a high shear mixing device and the slurry is introduced into a vessel. The slurry rises upwardly through the vessel. There is no substantial agitation of the fibers as they rise upward, and the pressure on the pulp is then gradually reduced. The maximum pressure difference is between 1 and 10 atmospheres. This is preferably done in a bleach tower having a height of between 40 and 300 feet.
"Generally speaking from about 5 to 120 minutes is sufficient. For the higher initial pressure provided by the higher tower the time can be reduced to a period of from about 1 minute to 60 minutes. With a 40 foot tower providing a pressure differential of roughly about 1 atmosphere about 30 to 60 minutes, preferably about 40 minutes is satisfactory."
The oxygenated pulp does not go directly to the tank. Between the mixer and the tank are a heat exchanger 5, a vent 7, and optionally a prepressurizing chamber 6.
The Rauma-Repola system is described in the Federal Republic of Germany Pat. No. 24 41 579, Mar. 13, 1975 and in Yrjala et al, "New Aspects in Oxygen Bleaching," dated Apr. 18, 1974. The system uses the Vortex mixer shown in FIGS. 2 and 3 of the patent. It is possible, by using either a number of passes through a single mixer or several mixers, to bleach the pulp in from 5 to 15 minutes.
Yrjala, et al. "A new reactor for pulp bleaching" Kemian Teollisuus 29, No. 12: 861-869 (1972) describes a chlorine reactor.
Reinhall U.S. Pat. No. 4,082,233 discloses a refiner having means for removing excess gas before the stock enters the refiner.